In a WDM transmission, several channels or transmission signals independent of each other are required to be sent over the same line consisting of an optical fibre, by multiplexing within the domain of the optical frequencies. The transmitted channels may be both digital and analog and are distinguishable from each other because each of them is associated with a specific frequency.
In such a transmission the different channels must be substantially equivalent to each other, that is none of them must be more or less privileged relative to the others, in terms of signal level or quality.
In the presence of amplifiers, in particular optical amplifiers, the amplifiers are required to substantially have the same response to all transmitted channels. In addition, in order to enable transmission of a high number of channels, the band in which the amplifier can operate is required to be wide.
Optical amplifiers are based on the properties of a fluorescent dopant, and in particular erbium, which is introduced as the dopant into an optical fibre core. In fact, erbium, excited by luminous pumping energy supply, exhibits a high emission in the wavelength range corresponding to the range of minimum light attenuation in silica-based optical fibres.
When an erbium-doped fibre, where erbium is maintained to an excited state, is passed through by a luminous signal having a wavelength corresponding to such a high emission, the signal causes transition of the excited erbium atoms to a lower level with a luminous emission stimulated to the wavelength of the signal itself, thereby generating a signal amplification.
Starting from the excited state, decay of the erbium atoms takes place also spontaneously, which generates a random emission constituting a "background noise", which overlaps the stimulated emission corresponding to the amplified signal.
The luminous emission generated by admitting luminous pumping energy to the "doped" or active fibre can occur at several wavelengths typical of the doping substance, thereby giving origin to a fluorescence spectrum in the fibre.
In order to achieve the greatest amplification of a signal by means of a fibre of the above type, together with a high signal/noise ratio suitable for a correct reception of the signal itself, in optical telecommunications, a signal is usually used which is generated by a laser emitter and has a wavelength corresponding to the maximum, in the intended band, of the fluorescence spectrum curve of the fibre incorporating the employed doping substance, or emission peak.
Erbium-doped fibres, on the other hand, have an emission spectrum with a peak of limited width, the features of which vary depending on the glass system into which erbium is introduced as the dopant, and a spectrum area of such a high intensity in a wavelength range contiguous to the above peak, within the wavelength range of interest, that the use of optical amplifiers for amplifying signals in a wide band is deemed to be possible.
Known erbium-doped fibres however, exhibit an uneven course of the emission spectrum. This uneven course affects the possibility of achieving a uniform amplification over the whole selected band.
In order to achieve a substantially "flat" gain curve, that is a gain as much constant as is possible at the different wavelengths, by eliminating noise sources due to a spontaneous emission, filtering elements may be used, such as those described for example in patents EP 426,222, EP 441,211, EP 417,441, filed in the name of the assignee of this application.
In such patents however, the amplifiers' behaviour in the presence of a wavelength division multiplexing is not described and, in addition, behaviour in the presence of several amplifiers connected with each other in cascade is not taken into account.
The emission spectrum profile greatly depends on the dopants present in the fibre core in order to increase the refractive index thereof, as shown for example in U.S. Pat. No. 5,282,079, in which the fluorescence spectrum of an alumina/erbium-doped fibre is shown to have a less marked peak and be at lower wavelengths than a germanium-erbium-doped fibre (which has a maximum at about 1532 nm). Such a fibre had a numerical aperture (NA) of 0.15.
In ECOC '93, ThC 12.1, pages 1-4, a fibre for an optical amplifier doped with Al and La and having a very low responsiveness to hydrogen is disclosed. The described Al-doped fibre has a numerical aperture (NA) of 0.16 and the Al--La-doped fibre has a numerical aperture (NA) of 0.30.
In ECOC '93, Tu 4, pages 181-184, optical amplifiers having erbium-doped fibres are disclosed. Experiments carried out with fibres the cores of which are doped with aluminium, aluminium/germanium and lanthanum/aluminium are described and the best results appear to be reached with Al/La-co-doped fibres.
In Electronics Letters, Jun. 6, 1991, vol. 27, No. 12, pages 1065-1067, it is pointed out that in optical amplifiers having an erbium-doped fibre, a co-doping with alumina enables a larger and flatter gain profile to be reached. The article describes amplifiers having an alumina-, germanium- and erbium-doped fibre as compared with amplifiers having a lanthanum-, germanium- and erbium-doped fibre. It is therein stated that the greatest gain flattening is obtained by the former.
In ECOC '91, TuPS1-3, pages 285-288 a fibre of the Al.sub. O.sub.3 --SiO.sub.2 type doped with Er and La is described for the purpose of obtaining a higher refractive index and reducing the formation of clusters containing erbium ions. The fluorescence and absorption spectra of the Er/La-doped fibre proved to be very similar to those of an erbium-doped Al.sub.2 O.sub.3 --SiO.sub.2 fibre. A numerical aperture (NA) of 0.31 was achieved as well as an erbium concentration of 23.10.sup.18 cm.sup.-3.
In ECOC '89, Post-Deadline Papers, PDA-8, pages 33-36, Sep. 10-14, 1989, an experiment made with twelve optical amplifiers connected in cascade using an erbium-doped fibre is disclosed. A single signal wavelength of 1.536 .mu.m was used and it is pointed out that signal wavelength control on the order of 0.01 nm is required for steady operation, in view of the fact that BER (Bit Error Rate) features rapidly decay on changing of the signal wavelength.
U.S. Pat. No. 5,117,303 discloses an optical transmission system comprising concatenated optical amplifiers that, based on the stated calculations, give a high signal/noise ratio, when operating in a saturated manner.
The described amplifiers have an erbium-doped fibre having an Al.sub.2 O.sub.3 --SiO.sub.2 core and the use of filters is described. The calculated performance is achieved at a single wavelength and a feeding signal in a wide wavelength band offering the same performance is not described.
U.S. Pat. No. 5,111,334 describes a multi-stage amplifier, in which the fibres in each stage are different from each other as regards length, dopant or base material, in order to obtain a maximum gain in a wide wavelength range. Provision is made for the use of a great number of stages, corresponding to the different signal wavelengths, in order to obtain a low ripple response.
According to that patent, provision is not made for the possibility of achieving a flat gain in a wide wavelength band with signals simultaneously fed by a single-type fibre having the same wavelength of maximum gain with varying of the fibre length.
In this connection it is pointed out that the erbium-doped fibres exhibit a single wavelength of maximum gain, which is independent of the fibre length. In addition, in that patent the problem of carrying out transmission through several amplifiers in cascade is not dealt with.
In IEEE PHOTONICS TECHNOLOGY LETTERS, vol. 4, No. 8, August 1992, pages 920-922, A. R. Chraplyvy et al, an amplified WDM system is disclosed in which the gain equalization is accomplished by means of information given by telemetry. In this system, performance in terms of signal/noise ratio (SNR) is equalized by an iterative signal adjustment at the emission, based on the signals at the reception. The return information is given by telemetry.
In JOURNAL OF LIGHTWAVE TECHNOLOGY, vol. 8, No. 9, September 1990, M. Maeda et al, the effects of mixing between signals in a multi-wavelength system, which is due to the non-linear character of the optical single-mode fibres is described. Line amplifiers connected in cascade are not provided.
The U.S. Pat. No. 5,088,095 and ELECTRONICS LETTERS, Mar. 28, 1991, vol. 27, No. 7, by the same author, describe a gain control method in an erbium-doped-fibre amplifier in which the amplifier is located in a laser loop configuration, with a feedback of a wavelength different from that of the signal to be amplified.
The feedback loop is provided for use under the operating conditions of the amplifier and such an application is not related to design criteria of the amplifiers.
In U.S. Pat. No. 5,280,383, a two-stage amplifier is disclosed in which the first stage operates under small-signal conditions and the second stage operates under saturation conditions, thereby providing a gain compression. This provides a reduction in the required pumping power.